Generally, a power amplifier is a device that converts an input signal with a small amount of energy into a similar output signal with a larger amount of energy. For example, a power amplifier could receive an input signal with a relatively small amplitude and convert it to an output signal with a relatively large amplitude. Radio transmitters, such as base stations, cell phones and other communication devices, often use these power amplifiers to transmit communication signals between remote devices.
An ideal power amplifier would be a totally linear device, meaning that as a drive signal to the amplifier is increased, the output signal delivered by the amplifier also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output.
In some applications, such as in mobile communication devices, efficiency (low-power design) is also an important consideration in addition to linearity. Referring now to FIG. 1, one can see an example of a somewhat inefficient power amplifier 100. This power amplifier 100 includes a power amplification stage 102, a circulator 104, and a band-pass filter 106 that has an output coupled to a load (RL). During operation, the power amplification stage 102 delivers a voltage over a wide bandwidth. The in-band frequency components flow through the circulator 104, pass through the pass band of the band-pass filter 106, and to the load RL, as indicated by line 108. The out-of-band frequency components, however, see a high impedance at the band-pass filter 106 and are reflected back towards the circulator 104, which dissipates the power in a dump resistor 110, as indicated by line 112. This leads to lost power and a power amplifier with a relatively low efficiency.
As will be appreciated from the above discussion and embodiments described herein, there is an on-going need for power amplifiers that exhibit linearity as well as high efficiency.